Bombora is developing a Wave Energy Converter (WEC) to extract commercial quantities of power from the ocean. Each unit is planned to be rated at 1.5MWe, being a similar scale to the average size of most onshore wind turbines currently deployed.
Surface gravity waves are created by wind (fetch) blowing on the ocean’s surface and forcing the water to move in an orbital fashion. The majority of deep water ocean wave energy is contained in a column of water generally to a depth of ½ the wavelength. As the waves pass to the near shore the energy is concentrated into a smaller column of water. The Bombora device is submerged, located on the seabed in the nearshore at shallow water depths. The Technology has a wide front, parallel with the waves. These features enable Bombora to be more accessible to and capture more of the water columns energy per unit deployed.
Submerged / Seabed location
The prime Bombora device is submerged and located on the seabed in the nearshore (5 – 15m water depth) environment. These features enable the device to be in a relatively protected environment yet still access a significant level of wave energy.
Many floating and submerged WECs, particularly offshore devices, require complicated and costly mooring or foundation designs. The Bombora device has been designed to be a sturdy, negatively buoyant body that rests directly against the sea floor to provide a natural and simple method of storm protection. A simple mooring or foundation system can be incorporated to provide a greater level of restraint against localised wave loads if required and to accommodate a range of seafloor bathymetry and geotechnical requirements. The mooring system can be installed by a dive team in a couple of days usually without the need for expensive piling, barges or jack up rigs. Geotextile bags (commonly used in high erosion beach preservation applications and artificial surfing reefs) can be utilised to provide a stable foundation platform, again being relatively simple to install. For rocky bottoms several point loading foundation systems are under consideration to increase the application locations, however more prospective sites will be cherry picked first.
Collisions with vessels are also avoided by being mounted on the seafloor. With its upper extremities being several meters under Mean Sea Level, it has good keel clearance for most recreational fishermen with larger commercial vessels being informed of its location through cardinal markers and Department of Transport notifications.
(Many other location and device configurations have been devised and covered in our patent application. Once developed for the near shore environment we envisage optimising the design for use in other locations and in conjunction with other renewable resources).
Streamlined design – surge and heave capture
The majority of deep water ocean wave energy is contained in a column of water generally to a depth of ½ the wavelength (i.e. around 40m deep). The water particles tend to move in an orbital fashion with the energy generally ‘rolling’ through the ocean. As the waves pass to the near shore, some energy is lost, but the remainder is concentrated into a smaller column of water with the particles moving in a more elliptical manner.
With these ‘rolling’ motions within the waves in mind, Bomboras designers have created a unique WEC structure with a novel inclined ‘ramp’ like feature that accentuates the water particle motions to extract more of the energy, at the same time as being ‘streamlined’ to reduce friction and unnecessary forces on the device.
The unique ramp like feature provides the ability to efficiently capture both heave and surge motions within the wave. This novel (patent pending) design impedes the wave’s surge (or forward) motion forcing the wave to heave (rise higher) accentuating the forces acting on the power capture elements of the device. The shape also restricts flow back over the structure during a wave trough, lowering the wave depth and emphasizing the effective height variation of the wave as it passes. This combined energy capture system has advantages over many other systems which tend to have only one degree of freedom or focus on one main component of the wave motion.
(Many shapes and device profiles have been devised and covered in our patent application. These will be further evaluated and developed during our next stages of development).
Almost all wave energy devices use some intermediate mechanism to capture energy from the wave and transfer it to a second fluid that drives a turbine and generator. Wave energy itself is quite complex, and the primary purpose of the intermediate mechanism is to be responsive to the wave and efficiently capture and transfer the energy to the intermediate fluid.
The UK Carbon Trust, in their Accelerating Marine Energy report (July 2011) identified there is a need for wave devices to exhibit better “coupling” with the wave to help reduce the cost of energy. Devices can do this by being lightweight and engaging well with the wave.
The Bombora device uses a simple, low cost, relatively light weight flexible membrane system to do this. The membrane has a series of individual cells below it, along its length separated by diaphragms. The membrane directly transfers the wave energy to pressurised air (pneumatic) within the device. Air also exhibits lightweight characteristics.
Many other devices use large, heavy, rigid bodies to interact with the wave, capturing the energy and transferring it to a liquid system. Both the heavy body and liquid system exhibit high inertia or lag and tend to be less responsive to the complex wave movements.
Flexible membrane WECs themselves are not new. Quite a lot of work was done in the 1970’s and 1980’s on these devices, with them being generally considered to have great low cost potential. Relatively little work had been undertaken on them in the period since, until Bombora’s Principals began their development and AWS picked up some of the principles of the popular offshore, ocean surface mounted circular SEA Clam device a few years ago.
Bombora’s device builds on the earlier near shore, submerged membrane device development, overcoming the wave capture limitations of those with its novel inclined or ‘ramped’ design and Vee shaped configuration.
Pneumatic power transfer
Bombora’s device operates much like a series of foot pumps, pressurising air into a common manifold system to a drive turbine and generator. As the wave passes over the device, it impresses upon the membrane (and individual diaphragm cells underneath) to create a sequence of positive system pressures in areas under the wave peak and negative system pressures in areas below the wave trough. These positive and negative pressures provide a pressure differential across the turbine and ultimately generate flow around the close loop circuit following a ‘source and sink’ principle. Check valves are introduced between the diaphragm cells to control the flow in one direction.
Low inertia, low friction
Utilising a light weight, flexible membrane and low inertia medium such as air allows the system to respond more in phase with the complex, real world wave motions, while minimising friction losses. The prime purpose of the fluid medium, whether it be air, water or hydraulic fluid is to efficiently collate and transmit energy, from the mechanism in contact with the wave, to a turbine to convert the energy to rotational power.
Bombora assessed alternative working mediums such as liquids and found that friction losses became substantial and the inertial phase lag (created from accelerating and decelerating the liquid) made the system slow and not responsive enough to effectively capture the passing wave energy flux over a broad range of operating conditions.
The effect can be thought of much like the difference you feel between sucking water or air in and out of a child’s rubber duck. The duck is much harder to fill and empty when held underwater, due to the friction and inertial effects of the water.
Wave energy is a variable, pulsating form of energy. The Bombora designers considered many ways to harness and ‘regulate’ (or rectify) it. By utilising the multi cell arrangement, simple check valves, common pressure and return manifolds, and the source and sink principles, a single directional airflow is created that tends to be naturally rectified. This allows a relatively even airflow to be presented to the turbine, and the turbine to be more optimised and less expensive than in bi-directional flow systems. (Some further investigation of bi-directional flow systems will be carried out as part of our on-going cost evaluations and are covered by our patent applications).
On-board PTO (turbo generator)
Power is extracted from the pneumatic (or ‘pressurised air’) circuit through an on-board power take off unit (PTO) or turbo-generator. The PTO is a simple device with an air turbine mounted directly onto the shaft of an electrical generator. The decision was made to embed the PTO on the device in the ocean to reduce overall cost and system efficiency losses. Although there can seem to be additional technical requirements and operating costs relative to shore mounted turbo-generator units, it was determined that the additional capital costs associated with the piping system and the friction and efficiency loss arising from pumping air or liquid to a shore mounted unit would far outweigh the impacts of an on-board turbo-generator.
The PTO is mounted in a modular housing unit. Quick coupling, wet mate connections for electricity and instrumentation (E&I) cables, and isolation packers for the airflow ducting allows for quick change out of the module for more detailed preventative or corrective maintenance.
Light wave operation
Much of the world’s useful ocean wave resource is contained in relatively small coastal waves. Due to the low friction and low inertia features the Bombora’s WEC, it can effectively absorb energy at low sea states. Designed around an average wave height of 1.4 meters, this will enable near year round energy production and the potential for dispatchable power.
The Bombora designers have considered safe operation and shutdown as a core input to the ultimate success of the technology. The device will have a number of safety features built in. From a turbine bypass circuit that will be completely automated during high seas (to enable energy extraction whilst still providing machinery protection) to the ability to evacuate the air circuit for complete shutdown during extreme seas, rogue waves or for easy maintenance. Being mounted on the seabed, the device also cannot ‘sink’ and its streamlined shape provides a level of inherent protection against adverse or unwanted wave forces.
Simple servicing solutions
With experience of having provided services to one of Australia’s most remote and isolated FPSOs and many remote, hard to access mine sites, the designers have downtime and maintenance duration minimisation at the forefront of mind. The Technologies key components incorporate cartridge type designs. This allows for “plug and play” style maintenance of key components (diaphragms, valves and PTO). Handling such equipment subsea requires specialist tools. Bombora is also developing unique automated maintenance equipment with interfaces incorporated into the technologies design. It is envisaged that these tools will be remotely operated and ground based (somewhat like a Caterpillar track based ROV). This feature will enable maintenance during higher energy sea states, thus increasing the available service windows and operational uptime.